Exposure control in an x-ray image detector

ABSTRACT

An x-ray detector apparatus comprises an array of detector pixels ( 20 ), each pixel ( 20 ) comprising a conversion element ( 26, 260 ) for converting incident radiation into a charge flow, a charge storage element ( 28 ) and a switching device ( 29 ) enabling the charge stored to be provided to an output of the pixel ( 20 ). A plurality of dose sensing pixels further comprise a dose sensing element ( 40, 50 ) during x-ray exposure results in a change in the charge stored on the charge storage element ( 28 ) and also results in a dose sensing signal being generated which can be read out from the pixel ( 20 ). The dose sensing pixels enable a dose signal to be obtained without reading the charges stored on the pixel charge storage elements, so that dose sensing can be carried out during exposure.

The invention relates to an X-ray detector and to an X-ray examinationapparatus, which uses the detector. The detector is for providing imagesignals as well as exposure control signals. In particular, theinvention relates to an X-ray examination apparatus in which exposuremeasurement circuitry is integrated with solid state X-ray detectorcircuitry, which enables real time control of the X-ray exposure duringan image acquisition process.

It is well known that the X-ray exposure of a patient should becontrolled as a function of the absorptivity of the tissue underexamination. For example, overexposed areas of high brightness may occurin the image, for example caused by X-rays which are not (or onlyhardly) attenuated by the object to be examined, for example a patient.Tissue having a low X-ray absorptivity, for example lung tissue, willprovide less attenuation and therefore requires less X-ray exposure toobtain an image of given contrast and to prevent saturation of the imagedetector.

Configurations of known X-ray examination apparatus are well known tothose skilled in the art. Typically, the apparatus includes an X-raysource for irradiating a patient to be radiologically examined, by meansof an X-ray beam. Due to local differences in the X-ray absorptivitywithin the patient, an X-ray image is formed. The X-ray detector derivesan image signal from the X-ray image. In a detector using an opticalsensor, the detector has a conversion layer or surface for convertingthe incident X-ray energy into optical signals. In the past, theseoptical signals have largely been detected by an image intensifierpick-up chain, which includes an X-ray image intensifier and atelevision camera.

A known X-ray examination apparatus of this type is disclosed in U.S.Pat. No. 5,461,658. This document additionally discloses an exposurecontrol system in which an auxiliary light detection system utilizeslocal brightness values in the optical image in order to adjust theX-ray source. This auxiliary light detection system includes a CCDsensor for locally measuring the brightness in the optical image. Theexposure control system derives a control signal from the measuredbrightness values, the control signal being used to adjust the X-rayapparatus in such a manner that an X-ray image of high diagnosticquality is formed and displayed, namely such that small details areincluded in the X-ray image and suitably visibly reproduced. The controlsignal controls the intensity and/or the energy of the X-ray beam andcan also be used to control the amplification of the image signal. Bothsteps influence the signal level of the image signal directly orindirectly.

More recently, the use of a solid state X-ray detectors have beenproposed. There are two basic configurations for such devices.

In a so-called “indirect” detector arrangement, the incident X-rayradiation is first converted into light. An array of photosensitivecells is provided, each comprising a light-sensitive element(photodiode), and a charge storage device (which may be a separateelement or it may be the self-capacitance of the photodiode).

In a so-called “direct” detector arrangement, an X-ray sensitivephotoconductor is used to convert the X-rays directly into electrons.Since the photoconductor has no self-capacitance, a capacitor isfabricated by thin film techniques to act as a charge storage device.

During X-ray exposure, the light incident on each cell is stored as alevel of charge on the charge storage device, to be read out at the endof the exposure period. The read out of charges stored effectivelyresets the image sensor, so this can only be carried out at the end ofthe X-ray exposure period. Thus, it is not possible to use the outputsignals from an image sensor of this type to control the exposure periodin real time, because such outputs are only available at the end ofexposure. The nature of the solid state image sensor device alsoprevents the type of feedback control described above using CCDs to beimplemented.

One possible way to achieve dose control is to analyse the obtainedimage, and then to repeat the image acquisition process with a differentexposure level. Of course, this increases the overall exposure of thepatient to potentially harmful X-ray radiation, and is also notappropriate for rapidly changing images, or where images from differentviewpoints are required in rapid succession.

External dose sensing arrangements have been proposed which areindependent of the solid state image detector, but these can degrade theimage quality. There is therefore a need for a dose sensing arrangementwhich enables real time dose control and which can be used with solidstate image sensors.

According to the invention, there is provided an X-ray detectorapparatus comprising an array of detector pixels, each pixel comprisinga conversion element for converting incident radiation into a chargeflow, a charge storage element and a switching device enabling thecharge stored to be provided to an output of the pixel, and wherein aplurality of dose sensing pixels further comprise a dose sensingelement, wherein charge flow from the conversion element during X-rayexposure results in a change in the charge stored on the charge storageelement and also results in a dose sensing signal being generated whichcan be read out from the pixel.

This detector is preferably used in an X-ray examination apparatuscomprising an X-ray source for exposing an object to be examined toX-ray energy. The detector receives an X-ray image after attenuation bythe object to be examined.

The apparatus may further comprise a phosphor conversion layer forconverting an incident X-ray signal into an optical signal, and theconversion element then comprises an optical sensor, such as aphotodiode. The charge storage element may then be a separate element inparallel with the photodiode, or it may comprise the self-capacitance ofthe photodiode.

Alternatively, the conversion element may comprise a photoconductor,which converts the X-ray radiation directly into an electron chargeflow.

The dose sensing pixels enable a dose signal to be obtained withoutreading the charges stored on the pixel charge storage elements, so thatdose sensing can be carried out during exposure.

The pixels may be arranged in rows and columns, with rows of pixelssharing a row address line and columns of pixels sharing a columnreadout line, wherein the charge storage element is connected in serieswith the switching device between a common electrode for all pixels andthe column readout line, the switching device being controlled by therow address line.

This is a known pixel configuration. In use, charge storage elements areall initially pre-charged. During exposure, the conversion element isisolated (because the switching device is turned off) and charge flowresults in partial discharge of the charge storage element. The level ofdischarge is measured at the end of the cycle (by measuring the flow ofcharge required to recharge the capacitor) and represents the level ofillumination. This known pixel configuration can be adapted in variousways to provide dose sensing pixels of the invention.

Defining a node between the charge storage element and the switchingdevice, the dose sensing element of the dose sensing pixels may comprisea further charge storage element connected between the node and a dosesignal readout line. As charges are supplied to this charge storageelement, the charge flow can be measured by a charge sensitive amplifierat the end of the dose signal readout line. However, the charge storedcan still be read out at the end of the exposure time, so that no imagesignal is lost.

Alternatively, the dose sensing element of the dose sensing pixels maycomprise a transistor connected between a dose electrode common for allthe dose sensing pixels and a dose signal readout line, the gate of thetransistor being connected to the node. In this arrangement, the voltageon the charge storage capacitor is supplied as a gate voltage. Thesource-drain current can then be sampled to obtain this gate voltage,which is a measure of the state of charge of the charge storage element,and therefore represents the preceding level of incident X-rayradiation. Again, the measurement of the dose sensing signal does notdestroy the image sensor signal on the charge storage element.

Preferably, the dose sensing signals for a plurality of dose sensingpixels are supplied to an individual dose signal readout line. Forexample, the dose signal readout lines may be parallel to the columnreadout lines and arranged alternately with the column readout lines.

The dose sensing pixels associated with an individual dose signalreadout line may be arranged in a block, and wherein pixel dose outputsin the block are connected together in columns with column lines, and asingle row connection line is provided for connecting together the pixeldose outputs of different columns in the block. This single rowconnection line enables the number of points at which dose signal linescross to be minimised, which reduces cross talk.

The dose signal readout lines may alternatively be parallel to the rowaddress lines and are then arranged alternately with the row addresslines.

Preferably, all pixels are dose sensing pixels. This enables all pixelsto have the same layout, which reduces image artifacts.

The invention also provides a method of using the X-ray examinationapparatus of the invention, the method comprising:

exposing the object to be examined with X-ray radiation;

monitoring output signals from selected dose sensing pixels during theexposure;

halting the X-ray exposure in response to the dose sensing signalmonitoring; and

reading out the charges stored on the charge storage elements to obtainan X-ray image.

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a known X-ray examination apparatus;

FIG. 2A shows a first known pixel layout for a solid state image sensorused in the apparatus of FIG. 1;

FIG. 2B shows a second known pixel layout for a solid state image sensorused in the apparatus of FIG. 1;

FIG. 3 shows a first modified pixel arrangement according to theinvention;

FIG. 4 shows a second modified pixel arrangement according to theinvention;

FIG. 5 shows a first grouping arrangement for dose sensing pixels of theinvention;

FIG. 6 shows a second grouping arrangement for dose sensing pixels ofthe invention;

FIG. 7 shows a third grouping arrangement for dose sensing pixels of theinvention;

FIG. 8 shows a fourth grouping arrangement for dose sensing pixels ofthe invention; and

FIG. 9 is a timing diagram for explaining the image acquisition methodof the invention.

FIG. 1 shows a known X-ray examination apparatus which includes an X-raysource 10 for irradiating an object 12 to be examined, for example apatient to be radiologically examined, by means of an X-ray beam 11. Dueto local differences in the X-ray absorption within the patient, anX-ray image is formed on an X-ray-sensitive surface 13 of the X-raydetector 14.

One known design of X-ray detector 14 uses a solid state optical imagesensor. The incident X-ray radiation is converted into light using aphosphor scintillator 13. This light can be detected by the solid-statedevice 14. Alternatively, an X-ray sensitive photoconductor can be usedto convert the X-rays directly into electrons.

FIG. 2 shows one known design for the solid state optical image sensor.The sensor comprises an array of pixels 20 arranged in rows and columns.Rows of pixels share a row address line 22, and columns of pixels sharea readout line 24. Each pixel comprises a photodiode 26 in parallel witha charge storage capacitor 28. This capacitor 28 may be a separatecomponent, or else it may simply comprise the self-capacitance of thephotodiode 26. This parallel combination is connected in series with athin film transistor 29 between a common electrode 30 and the columnreadout line 24 for that particular pixel. The pixel array is providedon a glass substrate 32. Row driver circuitry 34 provides signals forthe row address lines 22, and the column readout lines 24 provide anoutput from the substrate 32, and each column readout line 24 isassociated with a respective charge sensitive amplifier 36.

The function of the photodiode is to convert the incident radiation intoa flow of charge which alters the level of charge stored on thecapacitor. In the case of direct conversion of the radiation using aphotoconductor, the capacitor 28 is implemented as a separate thin filmcomponent, and again the level of charge stored is a function of theflow of charge from the photoconductor. FIG. 2B shows a known design ofsolid state direct X-ray detector. The same references are used as inFIG. 2A for the same components. The photoconductor 260 is biased to asuitable operating voltage and effectively replaces the photodiode 26 inthe arrangement of FIG. 2A.

In operation of the image sensor device, the capacitors 28 are allcharged to an initial value. This is achieved by the previous imageacquisition or else may be achieved with an initial reset pulse on allrow conductors 22. The charge sensitive amplifiers are reset using resetswitches 38.

During X-ray exposure, light incident on the photodiodes 26 causescharge to flow in the reverse-bias direction through the photodiodes.This current is sourced by the capacitors 28 and results in a drop inthe voltage level on those capacitors. Alternatively, the charge flowthrough the photoconductor 260 drains the charge from the capacitors 28.

At the end of X-ray exposure, row pulses are applied to each rowconductor 22 in turn in order to switch on the transistors 29 of thepixels in that row. The capacitors 28 are then recharged to the initialvoltage by currents flowing along between the common electrode 30 andthe column readout lines 24 and through the transistor switches. In theexample shown, these currents will be sourced by the charge sensitiveamplifiers 36, rather than flow to them. The amount of charge requiredto recharge the capacitors 28 to the original level is an indication ofthe amount of discharge of the storage capacitor 28, which in turn is anindication of the exposure of the pixel to incident radiation. This flowof charge is measured by the charge sensitive amplifiers. This procedureis repeated for each row to enable a full image to be recovered.

A problem with the use of solid-state image sensors of this type is thata pixel signal is only obtained during the read out stage, after theexposure has been completed. As will be apparent from the abovedescription, any read out of signals results in recharging of the pixelcapacitors 28, and effectively resets those pixels. Therefore, it is notpossible to take samples during the image acquisition process, and theimage sensor design does not therefore allow real-time exposuremeasurements to be obtained.

In accordance with the invention, dose sensing pixels are provided whichinclude a dose sensing element, which provides a dose sensing signalwhich can be read out from the pixel without resetting the charge storedon the pixel capacitor 28.

In the following description, optical detector pixels are shown withmodification to provide the dose sensing function of the invention.However, the invention applies equally to direct detection schemes suchas shown in FIG. 2B.

FIG. 3 shows a first modification to the basic pixel layout of FIG. 2Ato provide a dose sensing pixel of the invention. Throughout theFigures, the same reference numbers will be used for the samecomponents, and description of those components will not be repeated.

In addition to the components already described with reference to FIG.2A, the pixel further comprises a transistor 40 connected between acommon electrode 42 (common for all dose sensing pixels) and a dosesignal readout line 44. The gate of the transistor 40 is supplied withthe voltage on the pixel capacitor 28.

As will be explained in further detail below, a number of dose sensingpixels share the dose signal readout line 44.

The operation of this pixel configuration will now be described. Inconventional manner, the voltage on the pixel capacitor 28 is preset toa known level before the image acquisition process. Consequently, thegate of the dose sensing transistor 40 is also at this known potential.The common electrode 42 is at a potential which allows a quiescentcurrent to flow from a dose sensing current amplifier 46 through thetransistor 40 and to the common electrode 42, with the gate of thetransistor 40 being at this initial gate potential.

During X-ray exposure, the voltage on the pixel capacitor 28 changes,which changes the gate-source voltage of the transistor 40. The sourceof transistor 40 is connected to the common electrode 42. A change incurrent through the transistor 40 results, which is detected by the dosesensing current amplifier 46.

The connection of a number of pixels to a shared dose signal readoutline 44 results in the summation of the drain currents from thesetransistors. This summed drain current is then sensed by the currentamplifier 46.

Preferably, the amplifiers 46 provide only a small drain-source voltageto the dose sensing transistors 40. This allows the transistors 40 tooperate in the linear region, where the drain current depends linearlyon the gate-source voltage. Due to the linear dependency of this voltageon the accumulated charge on the pixel capacitance, and hence on thedose incident on the pixel cell, there is also a linear dependency ofthe drain current on the incident dose. A resistor can be added betweenthe source of transistor 40 and the common electrode 42, essentiallyforming a voltage-controlled current source. In this case, thedrain-source voltage does not need to be small compared to thegate-source voltage.

FIG. 4 shows a second modification of the pixel layout of FIG. 2A toprovide the dose sensing capability. In this pixel configuration, afurther capacitor 50 is provided between the dose signal readout line 44and the junction 52 between the switching transistor 29 and the parallelphotodiode/capacitor arrangement 26, 28.

As described above, during X-ray exposure, the photodiode 26 provides aflow of charge which is proportional to the dose incident on the pixel.Part of this charge is stored on the pixel capacitor 28, while the otherpart flows on to the dose sensing capacitor 50. This causes acorresponding flow of charge along the dose signal readout line 44. Thedose sensing charge sensitive amplifier 46 measures this flow of charge.As for the arrangement in FIG. 3, a number of pixels are associated withan individual dose signal readout line 44. The charge sensitiveamplifier 46 maintains a fixed potential at its input, so that crosstalk from one pixel cell to another does not arise.

At the end of the X-ray exposure, the pixels are read out inconventional way by switching on the pixel transistors 29 to allow acharge to flow along the column readout line 24 which recharges thepixel capacitor 28. However, charge also flows to the dose sensingcapacitor 50, so that charges flowing to or from the dose sensingcapacitor 50 during X-ray exposure are not lost, but are recovered whenthe image read out process takes place.

The dose sensing capacitor 50 is preferably smaller than the pixelcapacitor 29. This leaves most of the area occupied by the pixel cellfor the photodiode 26, ensuring a high efficiency of the pixel cell.Furthermore, this results in the total pixel capacitance being increasedonly slightly, with only a small consequent increase in switching noiseduring pixel read out.

An associated current amplifier 46 may be provided for each dose sensingsignal, or else a multiplexing switch arrangement may be used forselectively switching dose sensing signals to a shared current amplifier46.

As mentioned above, a number of dose sensing pixels are grouped togetherwith their dose sensing signal outputs being provided to a common dosesignal readout line. FIG. 5 shows one possible connection scheme forthis purpose, and using the pixel configuration of FIG. 4.

FIG. 5 shows four rows of pixels, with row address lines 22 ₁ to 22 ₄,and four columns of pixels, with column readout lines 24 ₁ to 24 ₄. Inthe example of FIG. 5 groups 60 of four pixels are provided, each with asingle dose signal readout line 62. The blocks 60 are arranged in rowsand columns, and between the columns of blocks, a number of dose signalreadout lines 62 are provided, corresponding in number to the number ofblocks 60 in each column.

A processing unit 64 collects the dose signals from each dose signalreadout line 62. It may be arranged to sum the dose signals of selecteddose sensing pixel blocks 60, and provide these as a first dose output66. Furthermore, a dose rate signal 68 may also be derived from theselected dose sensing pixel blocks 60, to indicate the dose per unittime.

As explained above, the exposure control is preferably carried out toprovide the best image contrast for an area of the image of particularinterest. Therefore, it is possible for the processing unit 64 toanalyse a particular pattern of blocks 60 of interest for the particularx-ray examination taking place.

Furthermore, different weights can be assigned to certain dose sensingpixel blocks 60 to obtain a weighted dose signal and dose rate signal.

The configuration shown in FIG. 5 provides clusters of dose signalreadout lines between columns of pixel blocks 60. This may providevisual artifacts in the final image, as it removes some of the pixelsymmetry. A number of alternative connection schemes will now bedescribed which enable the component and conductor layout for each pixelto be identical, thereby preventing the formation of these artifacts.

FIG. 6 again shows blocks 60 comprising four pixels which share a commondose signal readout line 62. In this example, the dose signal readoutlines 62 are parallel to the column readout lines 24 and the columnreadout lines and dose signal readout lines are arranged in an alternatesequence. The dose signal readout lines 62 extend the full height of thesensor array so that each dose signal readout line 62 passes through allpixels in a column, even if those pixels are not providing signals tothat particular line 62. For example, dose signal readout line 62 ₁ isshown to pass the full height of the array, but connections 70 areprovided only for the block of pixels 60 ₃, and not for the block ofpixels 60 ₁. Likewise, dose signal readout lines 62 ₂ passes throughboth blocks 60 ₁ and 60 ₃, although connections are only made to block60 ₁.

FIG. 7 shows a similar construction but with dose signal readout lines62 ₁ to 62 ₄ being arranged parallel with the row conductors 22 ₁ to 22₄, again with the dose signal readout lines 62 and the row conductors 22defining an alternating sequence.

One possible problem with the layouts of FIGS. 6 and 7 is that thenumber of crossing conductors may give rise to increased cross talkbetween dose sensing signals in different blocks 60. For example, inFIG. 6 the dose signal readout line 62 ₁ crosses signal conductorscarrying dose signals in the block 60 ₁ in two places, labelled 80 and82.

FIG. 8 shows a modification to the layout of FIG. 6 to reduce the numberof electrodes crossing the dose signal readout lines 62. In this case,pixel dose outputs 84 in each block 60 are connected together in columnswith column lines 86, and a single row connection line 88 is providedfor connecting together the pixel dose outputs 84 of different columnsin the block. This single row connection line 88 provides a singlecrossover 90 within each block of pixels 60 with the dose signal readoutlines 62 passing through the block.

FIG. 9 is a timing diagram used to explain the operation of the deviceduring image acquisition. The plots 100 are used to explain the normalimage read out operation, whereas plots 110 relate to the dose sensingfunction. The collection of plots 101 represent the row address signalsapplied successively to the rows of the array. Before X-ray exposure,each row is subjected to a reset pulse 102 which ensures that the pixelcapacitor 28 is charged to the known level. No further row addresssignals are applied during X-ray exposure 103, following which thecapacitors 28 of each row of pixels are recharged in turn, with a rowaddress pulse 104 being applied to each row in sequence. During each rowaddress pulse 104 charge sensitive amplifier outputs 105 for the fullset of amplifiers are obtained, and before the next row address pulse104, the charge sensitive amplifiers are reset using reset pulses 106.

During the X-ray exposure, the dose sensing current amplifiers 46provide analogue signals 112 indicating the level of illumination to therespective block of dose sensing pixels. These analogue signals aresampled to provide signals 114 which can be analysed using digitaltechniques to obtain exposure information. When a given condition hasbeen reached, analysis of the sampled outputs 114 results in terminationof the X-ray exposure period 103 which is followed by the read out stage115. The X-ray exposure may be pulsed, and the exposure control thendictates when the X-ray exposure ceases. The analogue signals 112 mayinstead be processed in the analogue domain without A/D conversion(sampling).

In the examples described above, the dose sensing pixels are described,in each case, as forming a block of 2×2 pixels. Of course, this is notnecessarily the case, and in fact the dose sensing pixels will begrouped in much larger groups. To enable the implementations of FIGS. 6to 8, there will only be as many dose sensing pixel blocks as there arerows or columns of the array. Taking an array with an equal number n ofrows and columns, the smallest possible size of the pixel block is n×npixels.

Of course, the array will not necessarily have the same number of rowsand columns, and indeed the pixel blocks which share a common dosesensing signal output will not necessarily be square.

The manufacturing processes involved in forming the solid state devicehave not been described. The additional components provided by theinvention can be integrated into existing pixel cells using the thinfilm techniques applied for those cells. Typically, such devices areamorphous or polycrystalline silicon devices fabricated using thin filmtechniques.

Although an additional dose sensing element is incorporated into thepixel design, a single photodiode is being used in each pixel to providethe image acquisition function as well as the dose sensing function.Similarly, the same scintillator (phosphor layer for X-ray-to-lightconversion) is used for image acquisition and dose sensing. Thus, thespectral matching between image sensor and dose sensor is perfect, whichcannot be achieved with external dose sensing devices. The slightly morecomplex pixel configuration will result in only a minor cost increase,as the same thin film deposition techniques will be employed as for theexisting pixel configuration.

The additional connections between pixels required in the pixel blocks60 are likely to require the solid state device to be manufactured in amultilevel configuration.

To illustrate the possible configurations, the capacitor implementationof FIG. 4 has been illustrated in each of FIGS. 5 to 8. The skilledaddressee will understand, of course, that the configurations of FIGS. 5to 8 may be applied equally to the individual transistor implementationrepresented in FIG. 3.

Various modifications will be apparent to those skilled in the art.

What is claimed is:
 1. An X-ray detector apparatus comprising an arrayof detector pixels, each pixel comprising a conversion element forconverting incident radiation into a charge flow, a charge storageelement and a switching device enabling the charge stored to be providedto an output of the pixel, and wherein a plurality of dose sensingpixels further comprise a dose sensing element, wherein charge flow fromthe conversion element during X-ray exposure results in a change in thecharge stored on the charge storage element and also results in a dosesensing signal being generated which can be read out from the pixelwithout resetting the charge of the charge storage element.
 2. Apparatusas claimed in claim 1, further comprising a conversion layer forconverting an incident X-ray signal into an optical signal, and whereinthe conversion element comprises an optical sensor.
 3. Apparatus asclaimed in claim 2, wherein optical sensor comprises a photodiode. 4.Apparatus as claimed in claim 3, wherein the charge storage elementcomprises the self-capacitance of the photodiode.
 5. Apparatus asclaimed in claim 1, wherein the conversion element comprises aphotoconductor.
 6. Apparatus as claimed in claim 1, wherein the dosesensing signals for a plurality of dose sensing pixels are supplied toan individual dose signal readout line.
 7. Apparatus as claimed in claim6, wherein the pixels are arranged in rows and columns, with rows ofpixels sharing a row address line and columns of pixels sharing a columnreadout line, and wherein the dose signal readout lines are parallel tothe column readout lines and are arranged alternately with the columnreadout lines.
 8. Apparatus as claimed in claim 7, wherein the dosesensing pixels associated with an individual dose signal readout lineare arranged in a block, and wherein pixel dose outputs in the block areconnected together in columns with column lines, and a single rowconnection line is provided for connecting together the pixel doseoutputs of different columns in the block.
 9. Apparatus as claimed inclaim 6, wherein the pixels are arranged in rows and columns, with rowsof pixels sharing a row address line and columns of pixels sharing acolumn readout line, and wherein the dose signal readout lines areparallel to the row address lines and are arranged alternately with therow address lines.
 10. Apparatus as claimed in claim 1, wherein allpixels are dose sensing pixels.
 11. An X-ray examination apparatuscomprising: an X-ray source for exposing an object to be examined toX-ray energy; and an X-ray detector for receiving an X-ray image afterattenuation by the object being examined, the X-ray detector comprising:an array of detector pixels, each pixel comprising a conversion elementfor converting incident radiation into a charge flow, a charge storageelement and a switching device enabling the charge stored to be providedto an output of the pixel, and wherein a plurality of dose sensingpixels further comprise a dose sensing element, wherein charge flow fromthe conversion element during X-ray exposure results in a change in thecharge stored on the charge storage element and also results in a dosesensing signal being generated which can be read out from the pixelwithout resetting the charge of the charge storage element.
 12. Anapparatus as claimed in claim 1, wherein the pixels are arranged in rowsand columns, with rows of pixels sharing a row address line and columnsof pixels sharing a column readout line, wherein the charge storageelement is connected in series with the switching device between acommon electrode for all pixels and the column readout line, theswitching device being controlled by the row address line.
 13. Apparatusas claimed in claim 12, wherein a node is defined between the chargestorage element and the switching device, and wherein the dose sensingelement of the dose sensing pixels comprises a further charge storageelement connected between the node and a dose signal readout line. 14.Apparatus as claimed in claim 12, wherein a node is defined between thecharge storage element the switching device, and wherein the dosesensing element of the dose sensing pixels comprises a transistorconnected between a dose electrode common for all the dose sensingpixels and a dose signal readout line, the gate of the transistor beingconnected to the node.
 15. A method of X-ray examination using anapparatus comprising an X-ray source for exposing an object to beexamined to X-ray energy and an X-ray detector for receiving an X-rayimage after attenuation by the object to be examined, the X-ray detectorcomprising an array of detector pixels, each pixel comprising aconversion element for converting incident radiation into a charge flow,a charge storage element and a switching device enabling the chargestored to be provided to an output of the pixel, wherein a plurality ofdose sensing pixels further comprise a dose sensing element, whereincharge flow from the conversion element during X-ray exposure results ina change in the charge stored on the charge storage element and alsoresults in a dose sensing signal being generated which can be read outfrom the pixel without resetting the charge of the charge storageelement, the method of controlling the X-ray examination apparatuscomprising the steps of: exposing the object to be examined with X-rayradiation; monitoring output signals from selected dose sensing pixelsduring the exposure; halting the X-ray exposure in response to the dosesensing signal monitoring; and reading out the charges stored on thecharge storage elements to obtain an X-ray image.